Fourier-plane photonics package

ABSTRACT

A photonics package, and methods for its use are disclosed. In one configuration, a collimating lens is disposed between a photonics device and a ferrule containing two optical fibers. Preferably, one of the fibers delivers an optical signal to the photonics device, and the other fiber receives an optical signal from the photonics device. The fibers within the dual-fiber ferrule are located off of the optical axis of the lens so that light emanating from the signal-delivering fiber will be imaged onto the photonics device at a slight angle from the normal and may be reflected at the same angle for coupling into the signal-receiving fiber. Preferably, the photonics device is situated at the Fourier plane to facilitate coupling reflected light into the signal-receiving fiber. The function of the photonics package varies with the included photonics device. For example, the package can function as a data receiver, a data transmitter and a data transceiver by incorporating, respectively, a photodetector, an optical modulator, and a transceiver. The photonics package, which can be integrated in optical communications networks, allows for incoming and outgoing signals to be handled on separate fibers, obviating the need for a splitter as required in one fiber systems. A decrease in signal loss throughout the optical communications system can thus be realized.

STATEMENT OF RELATED APPLICATIONS

The present application is related to "METHODS AND ARRANGEMENTS FORDUPLEX FIBER HANDLING", filed Jul. 26, 1996 as Ser No. 08/688,178,inventors Mark D. Feuer and Joseph E. Ford; "WAFER LEVEL INTEGRATION OFAN OPTICAL MODULATOR AND Ill-V PHOTODETECTOR", filed Jul. 23, 1996 asSer. No. 08/685,294, inventors John E. Cunningham, Joseph E. Ford, KeithWayne Goossen and James A. Walker; and, "METHOD AND ARRANGEMENT FOR ACOMBINED MODULATOR/PHOTODETECTOR", filed Jul. 5, 1996 as Ser. No.08/675,980, inventors David J. Bishop, Keith Wayne Goossen and James A.Walker. Each of the aforementioned applications is assigned to thepresent assignee.

FIELD OF THE INVENTION

The present invention relates generally to packaging photonics devices.

BACKGROUND OF THE INVENTION

Network architectures for two-way optical fiber communications to thehome have been proposed. Cost targets must be achieved for sucharchitectures to be implemented. Wavelength-Division-Multiplexed (WDM)network architectures, for example, have been proposed that use opticalmodulators, rather than expensive wavelength-stabilized sources, at eachhome. The optical modulators are powered by a shared laser source at acentral office.

Surface normal optical modulators operating in a reflection mode("reflective modulators"), that is, modulators that operate byreflecting, or not reflecting, an incident optical signal, may be usedin such networks. These modulators can be packaged by butt-coupling themto a single mode fiber. In such an arrangement, the reflected datasignal is carried in the same fiber that supplied the incident opticalsignal. For processing, the reflected data signal is separated from theincident signal, such as by passing the signal carrying fiber through a2×2 splitter. The splitter adds complexity to the system and can cause 6dB of intrinsic loss; 3 dB on each pass.

Thus, there is a need for a package for a reflective modulator that, inconjunction with the network architecture, reduces power losses.

SUMMARY OF THE INVENTION

A photonics package, and methods for its use and fabrication, aredisclosed. In one illustrative embodiment, a lens is disposed between aferrule containing two optical fibers ("dual-fiber ferrule") and aphotonics device. Preferably, one of the fibers ("the input fiber")delivers an optical signal to the photonics device, and the other fiber("the output fiber") receives an optical signal from the photonicsdevice. The lens is appropriately spaced from the dual-fiber ferrule forcollimating light. The fibers within the dual-fiber ferrule are offsetfrom the optical axis of the lens so that light emanating from the inputfiber will be received by the modulator at a slight angle from thenormal to the modulator. The angled incidence of the optical signal uponthe photonics device results, in preferred embodiments, in the signalbeing reflected toward and imaged onto the output fiber. Preferably, themodulator is situated at the Fourier plane of the lens to facilitatecoupling reflected light into the output fiber.

A photonics package according to the present invention has a variety ofuses as a function of the specific photonics device included within thepackage. Without limitation, the package can function as a datareceiver, a data transmitter and a data transceiver.

In addition, the package may be advantageously integrated in opticalcommunications systems. For example, in an illustrative embodiment, thephotonics package, which is incorporated into each one of a plurality ofnetwork units, receives information transmitted from a central office toeach network unit and encodes information on an optical signal fortransmission back to the central office.

Conventional photonics packages use a single fiber for delivering afirst optical signal to a photonics device within the package, and forreceiving a second optical signal from the photonics device fortransmission out of the package. A photonics package according to thepresent invention, however, allows for receiving a first signal througha first fiber, and transmitting a second signal out of the packagethrough a second fiber. As such, when using a photonics packageaccording to the present invention, the signals do not have to beseparated for processing. A splitter for separating the signals, whichis typically required in single fiber systems, is therefore not requiredin optical communications systems using the present photonics package.This results in a decrease in signal loss throughout the communicationssystem, and a decrease in system complexity and cost.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the invention will become more apparent from thefollowing detailed description of specific embodiments thereof when readin conjunction with the accompanying drawings, in which like elementshave like reference numerals and in which:

FIG. 1 is a first illustrative embodiment of a photonics package 1aaccording to the present invention;

FIG. 2a shows an exemplary dual-fiber ferrule;

FIG. 2b illustrates the path of an optical signal through the photonicspackage of FIG. 1;

FIG. 3 is a second illustrative embodiment of a photonics package 1baccording to the present invention;

FIG. 4 is a cross-sectional side view of an exemplary embodiment of amicromechanical optical modulator suitable for use in conjunction withthe present invention;

FIG. 5 is a top-view of the modulator of FIG. 4;

FIG. 6 is an exemplary embodiment of a combined opticalmodulator/photodetector for use in conjunction with the presentinvention;

FIG. 7 is an exemplary embodiment of a wafer-level-integrated opticalmodulator/photodetector for use in conjunction with the presentinvention;

FIGS. 8a-8c show three exemplary embodiments of using a filter forwavelength selection in conjunction with the present invention; and

FIG. 9 illustrates an embodiment of an optical communications systemusing a photonics package according to the present invention.

DETAILED DESCRIPTION

FIG. 1 shows a first illustrative embodiment of a photonics package 1aaccording to the present invention. In the illustrative embodiment, thephotonics package 1a includes a rigid, impact resistant sleeve 3 formedfrom a precisely shapeable material. In a preferred embodiment, thesleeve 3 is ceramic. The sleeve can have any convenient shape, e.g.,cylindrical, rectangular, and so forth. In an alternate embodiment, thesleeve 3 can be configured similarly to sleeves used for rotary slices,which typically include a beryllium copper sleeve shaped as a triangularprism with three glass rods in the creases as guide pins. The sleeve 3receives, at a first end 4, a first optical fiber 7 and a second opticalfiber 9. As shown in detail in a later Figure, the optical fiber 7 canbe used to deliver an optical signal 25 to the package 1a, and morespecifically to a photonics device 15, while the fiber 9 can be used forreceiving an optical signal 26 from the photonics device 15.

It will be appreciated that the optical fibers 7 and 9 must be retainedin a specific location with respect to other optical components withinthe system, as described in more detail below. According to anillustrative embodiment of the present invention, such positioning isachieved by a dual-fiber ferrule 5.

As shown in FIG. 2a, the dual-fiber ferrule 5 consists of a bore 82located along the longitudinal symmetry axis C--C of a retaining member80. The retaining member 80 is made from a rigid, stable materialcapable of being precisely formed into a desired shape. Preferably, theretaining member 80 is a ceramic. Optical fibers 7 and 9, with plasticcoating layers removed, i.e., the fibers 7 and 9 as received compriseonly a fiber core 7a, 9a and a cladding layer 7b, 9b, are received bythe bore 82.

In the dual-fiber ferrule 5 shown in FIG. 2a, the bore 82 is shown to beellipsoidal in cross section. Such a shape provides a single definedrotational orientation of the fiber cores. In the exemplary photonicspackage 1a, such definition is not required, so that the bore 82 can beround, as well. The size of the bore 82 is large enough to accept thetwo fibers in a tight fit.

Further embodiments and description of a dual-fiber ferrule suitable foruse in conjunction with the present invention is described in "METHODSAND ARRANGEMENTS FOR DUPLEX FIBER HANDLING", filed Jul. 26, 1996 as Ser.No. 08/688,178, assigned to the present assignee. That patentapplication, and any other patents, patent applications or publicationsmentioned in this specification are incorporated by reference herein.

The sleeve 3 receives, at a second end 6, the photonics device 15, whichis preferably disposed on a device mount 13. The device mount 13 can bean electrical header, for example, which provides electrical connectionbetween the photonics device 15 and processing electronics, not shown,located outside the package 1a. Electrical connection is provided byelectrical contacts 19. In the case of an electrical header, thecontacts 19 are typically pins.

It will be appreciated that the optical signal 25 from the optical fiber7 cannot be directed along a path normal to the photonics device 15. Insuch a case, the reflected signal 26 would be returned to the fiber 7.The optical signal 25 can, however, be directed to the photonics device15 at an appropriate angle so that the reflected optical signal 26 isimaged into the fiber 9. Thus, disposed within the sleeve 3 between theoptical fibers 7 and 9 and the photonics device 15 is an imaging device.In the illustrative photonics package 1a, the imaging device is a singlelens 11a. The lens 11a is used for imaging the optical fiber 7 into thefiber 9. Suitable lenses 11a include, without limitation, gradient index(GRIN) lenses, ball lenses and molded lenses, such as, for example,injection molded lenses.

As shown in FIG. 2b, the lens 11a is positioned a distance, d, from theoptical fibers 7 and 9, equal to the focal length of the lens 11a. Whenso positioned, the lens 11a will collimate the optical signal 25. Thefibers cores 7a and 9a are equidistant from the optical axis A--A of thelens 11a. The photonics device 15 is located at the Fourier plane B--B.As will be appreciated by those skilled in the art, the Fourier plane isessentially the back focal plane of a lens. A collimated beam entering alens would be focused to a point on a surface located at the Fourierplane. See Goodman, Introduction to Physical Optics, Chapter 5, "FourierTransforming and Imaging Properties of Lenses," (McGraw-Hill, 1968) fora mathematical definition.

It is particularly advantageous to place a reflective photonics device15 in the Fourier plane; doing so creates a telecentric optical systemupon two passes through the lens. A telecentric system is defined as onein which the entrance pupil and/or the exit pupil is located atinfinity. See Smith, Modern Optical Engineering, Chapter 6, Section 6,(McGraw-Hill, 1990). In the context of a fiber optic system,telecentricity means that the optical beam incident on the output fiberwill match the optimum incidence angle, resulting in optimized coupling.Thus, the reflected optical signal 26 will be imaged, via the lens 11a,into the optical fiber 9 with high efficiency.

Preferably, the device for retaining the optical fibers 7 and 9, such asthe dual-fiber ferrule 5, the photonics mount 13 and the sleeve 3 areformed so that they provide passive alignment for the optical fibers 7,9 and the photonics device 15. That is, the aforementioned componentsare designed such that when the photonics package 1a is assembled, theoptical signal 25 from the optical fiber 7 will be optically alignedwith the optical fiber 9.

In other embodiments, a photonics package according to the presentinvention can be actively aligned, such as by tilting the modulator ormoving the lens 11a and the photonics device 15 with respect to theoptical fibers 7, 9. In such embodiments, the various components areheld in fixtures so they can be moved as described above. Once thecomponents are optically aligned, they can be retained in position byvarious optical packages known to those skilled in the art. It will beappreciated that the sleeve 3 of the photonics package 1a is not used insuch embodiments.

There are relatively stringent tolerances on lens centration and fiberpositioning. These tolerances are achievable due to the symmetry of thedual-fiber ferrule 5 and GRIN or ball lens fabrication. Grin lens polishangle tolerance, which is expected to be much less than 1 degree, maynot be achievable using standard techniques, such as setting a batch oflenses in wax and group polishing the lenses. Achieving such tolerancesmay require using a polishing jig with holes drilled to accept a GRINlens and for holding the lens perpendicular to the optic axis. Using aspherical ball lens guarantees lens centration and eliminates theconcern with the polish angle of the GRIN lens. Device positioning andtilt tolerances are achievable using a conventional header mountingtechnique with a sufficiently large device die size. The gap tolerancesbetween the dual-fiber ferrule 5 and the lens 11a, and the photonicsdevice 15 and the lens 11a are on the order of several microns (μm) andare readily achievable.

A second illustrative embodiment of a photonics package 1b according tothe present invention is shown in FIG. 3. In the package 1b, the imagingdevice consists of two microlenses 12a and 12b. The microlenses 12a and12b are separated from the ends of the optical fibers 7 and 9 by a smallgap. It is within the capabilities of those skilled in the art tofabricated the microlenses 12a and 12b. For example, the microlenses canbe fabricated by depositing a refractive layer on a clear substrate andforming appropriately spaced spherical surfaces in the refractive layerusing, for example, photolithographic techniques. See, for example, D.R. Purdy, "Fabrication of Complex Micro-Optic Components using HalftoneTransmission Masks to Photosculpt Positive Resist," EOS Top. Mtg. Dig.S., Vol. 2, (1993). The substrate can then be diced and placed in closeproximity to the fiber ends. Precise alignment of each microlens 12a and12b to the fibers is required. In preferred embodiments, asphericcorrection is used to reduce signal loss from microlens aberrations.Such corrections are within the capabilities of those skilled in theart, and may be accomplished by using a diffractive microlens. Chromaticaberrations, especially in diffractive microlenses, may restrict theusable wavelength bandwidth.

Photonics packages according to the present invention, such as theillustrative packages 1a and 1b, have a variety of applications,depending upon the particular photonics device 15 included within thepackage. In one embodiment, the present photonics package functions as adata transmitter. In preferred embodiments of the photonics package as adata transmitter, the photonics device 15 is an optical modulator 15a.Either semiconductor optical modulators, such as multiple quantum wellmodulators, or micromechanical modulators may suitably be used. Anexemplary multiple quantum well modulator is described in Cunningham etal., "Reflectivity from Multiple Quantum Well Modulators with ContrastRatio of 22:1 at 1.55 μm," Conference on Lasers and Electro-Optics 9,1996, OSA Tech. Digest Series, p. 487. An exemplary embodiment of amicromechanical modulator 15a suitable for use in conjunction with thepresent invention is shown in FIGS. 4 and 5.

As shown in FIG. 4, which is a cross-sectional view through line DD inFIG. 5, the modulator 15a comprises a substrate 10 and a membrane 14having one or more layers, such as the layers 14a and an optional layer14b. The membrane 14 and the substrate 10 are spaced from each otherdefining a gap 20. As shown in FIG. 5, which is a plan view of themodulator 15a, the membrane 14 is suspended over the substrate 10 bysupport arms 24. The supports arms 24 are in turn supported by anonconductive support layer 16. In other embodiments, discrete supportarms 24 are not present. Rather, the membrane 14 itself overlaps thenonconductive support layer 16.

If the membrane 14 is not electrically conductive, a layer 28 ofconductive material, such as, without limitation, gold or otherconductive metals or alloys thereof, can be disposed on the membranelayer 14a. If the layer 28 is not transparent at the operatingwavelength of the modulator 15a, then an optical window 27 must bedefined with the layer 28.

The membrane 14 and the substrate 10, which are electrically isolatedfrom one another, are electrically connected to a controlled voltagesource 29. Applying a voltage across the membrane 14 and substrate 10generates an electrostatic force that moves the membrane toward thesubstrate. As the membrane 14 moves, the size of the gap 20 changes, andso does the reflectivity of the modulator 15a. The change inreflectivity of the modulator 15a alters the measured amplitude of anoptical signal reflected from the modulator. The changing reflectivityof the modulator 15a may thus be used to modulate an optical signal.

In the modulator 15a, a large change in reflectivity can be obtained ifthe following two conditions are met. First, the layer 14a has athickness that is one-quarter of a wavelength, λ, of the optical signalbeing processed ("the operating wavelength"), as measured in the layer.And second, the layer 14a has a refractive index, n_(m), that is aboutequal to the square root of the refractive index, n_(s), of thesubstrate 10. Given those parameters, the modulator 15a will be highlyreflective when the position of the membrane 14 is such that the gap 20is an odd integer multiple of one-quarter of the operating wavelength,that is, mλ/4 where m is odd. Conversely, the modulator 15a will exhibitminimal reflectivity, i.e., be transmissive, when the gap 20 is zero oran even integer multiple of one-quarter of the operating wavelength,that is, mλ/4 where m is even or zero.

For maximum modulator contrast, the modulator 15a is fabricated, i.e.,the gap 20 is sized, so that in the absence of an applied voltage, themodulator will exhibit its minimum or maximum reflectivity. As describedabove, this occurs when the gap 20 is an integer multiple of λ/4. Whenbiased, the membrane 14 preferably moves a distance of λ/4, so that thegap 20 is still at some multiple of λ/4. As such, the modulator exhibitseither maximum or minimum reflectivity in its biased mode, as well.

Thus, in embodiments in which the photonics device 15 is the modulator15a, the modulator receives the optical signal 25 from the optical fiber7 and returns a reflected optical signal 26, or not, to the opticalfiber 9, depending upon the state of the modulator.

Further non-limiting descriptions of an optical modulator 15a suitablefor use in conjunction with the present invention, including methods formaking it and other embodiments thereof are provided in U.S. Pat. No.5,500,761, and co-pending U.S. patent applications Ser. No. 08/283,106filed Jul. 29, 1994, Ser. No. 08/578,590 filed Jun. 7, 1995, Ser. No.08/479,476 filed Jun. 7, 1995, Ser. No. 08/578,123 filed Dec. 26, 1995,Ser. No. 08/565,453 and Ser. No. 08/597,003.

In another embodiment, a photonics package according to the presentinvention can function as a receiver. In preferred embodiments of thephotonics package as a receiver, the photonics device 15 is aphotodetector 15b. Suitable photodetectors 15b for use in conjunctionwith the present invention include, without limitation, photoconductors,photodiodes, avalanche photodiodes, phototransistors, heterojunctionphotodiodes, P-I-N multiple quantum well detectors and metal-insulatorIII-V photodiodes. The operation and fabrication of such photodetectorsare known to those skilled in the art.

In embodiments wherein the photonics package is functioning only as areceiver, the output fiber, i.e., the fiber 9, might not receive anoptical signal.

In an additional embodiment, a photonics package according to thepresent invention can function as a data transceiver, including both areceiving and a transmitting element. In such an embodiment, thephotonics device 15 is preferably a combined opticalmodulator/photodetector 15c or a wafer-level-integrated opticalmodulator/photodetector 15d. In such a photonics package, the opticalfiber 7 delivers the optical signal 25 to the combined opticalmodulator/photodetector 15c or the wafer-level-integrated opticalmodulator/photodetector 15d, and such devices send a return signal, suchas the optical signal 26, to the optical fiber 9. An exemplary combinedoptical modulator/photodetector 15c is illustrated in FIG. 6.

The exemplary combined optical modulator/photodetector 15c consists of amodulator chip 30a attached to a photodetector chip 40a. The modulatorchip 30a includes a substrate 34a having a first surface 33a and asecond surface 35a. Preferably, the substrate 34a is silicon, but, aswill be appreciated by those skilled in the art, other semiconductorstransparent at the operating wavelengths may suitably be used. Anoptical modulator 32a, is located along the first surface 33a of thesubstrate 34a. Contacts or wire bond pads 36a and 37a are in electricalcontact with a controlled voltage source, not shown, and are also inelectrical contact, respectively, with a feature of the modulator 32aand the substrate 34a. The optical modulator 32a can suitably beembodied as the modulator 15a described above.

The photodetector chip 40a includes a III-V substrate 44a having a firstsurface 43a. The III-V substrate 44a is preferably indium phosphide(InP) for optical communications applications, but may suitably be otherIII-V semiconductors, such as gallium arsenide (GaAs) in otherembodiments. A photodetector 42a is located along the first surface 43aof the III-V substrate. The photodetector 42a can suitably be embodiedas the photodetector 15b described above. The photodetector chip 40a canbe electrically connected to equipment, not shown, for processing andreceiving the electrical signal generated by the photodetector, throughcontacts or wire bond pads 46a and 47a.

In operation, the combined optical modulator/photodetector 15c receivesa downstream information-carrying optical signal, such as the opticalsignal 25 from the fiber 7. During a first time period, the modulator32a is placed in at least partially transmissive mode so that a firstportion of the signal 25 is absorbed by the photodetector 42a. During asecond time period, the optical modulator 32a encodes upstreaminformation upon the signal 25 creating the optical signal 26 which isreceived by the fiber 9. Such information encoding is accomplished by acontrolled variation of the reflectivity of the modulator 32a.

Further description of a combined optical modulator/photodetector 15csuitable for use in conjunction with the present invention, includingmethods for making it and other embodiments thereof, are provided in"METHOD AND ARRANGEMENT FOR A COMBINED MODULATOR PHOTODETECTOR," filedon Jul. 5, 1996 as Ser. No. 08/675,980.

An exemplary wafer-level integrated (WLI) opticalmodulator/photodetector 15d is shown in FIG. 7. The WLImodulator/photodetector 15d includes a modulation region 32b and aphotodetection region 42b that are formed on opposed surfaces 38 and 39,respectively, of an off-axis silicon substrate or wafer 40. Themodulation region 32b can suitably be embodied as the modulator 15adescribed above. A first and second wire from a controlled voltagesource, not shown, are bonded to bond pads or contacts 36b and 37b toplace the controlled voltage source in electrical connection with themodulator region.

In a preferred embodiment, the photodetection region 42b is disposed ona buffer layer 44 situated on the surface 39 of the wafer 40, ratherthan directly on the surface 39. The buffer layer 44 provides alattice-mismatch relaxation region between the first III-V layer,typically InP in communications applications, and the off axis substrate40. The detection region 42b can suitably be embodied as thephotodetector 15b mentioned above. A surface contact 46b on thephotodetection region 42b provides electrical contact to the top layerof the photodetection region, which, is typically either a n- or ap-doped layer. The other contact can be provided by the substrate 40.

The WLI optical modulator/photodetector 15d operates in substantiallythe same manner as the combined optical modulator/photodetector 15c.

Further description of a WLI optical modulator/photodetector 15dsuitable for use in conjunction with the present invention, includingmethods for making it and other embodiments thereof, are provided in"WAFER-LEVEL-INTEGRATION OF AN OPTICAL MODULATOR AND III-VPHOTODETECTOR," filed on Jul. 23, 1996 as Ser. No. 08/685,294.

In a further embodiment of a photonics package according to the presentinvention, the photonics package also includes a filter 17 forwavelength selection or wavelength drop. When placed between the lens11a and the photonics device 15 of the photonics package 1a, the filter17 will allow only light of a predetermined wavelength to reach thephotonics device. As is known to those skilled in the art, such as afilter can be embodied as a planar reflective surface or dielectricmirror comprising a plurality of dielectric layers selected to reflectspectral components having other than a predetermined wavelength.

The filter 17 can be located in several positions within a photonicspackage according to the present invention. In one embodiment, thefilter 17 can be disposed on the imaging device 11 provided that thesurface on which the filter 17 is disposed is flat. As such, the filter17 can be suitably disposed on a GRIN lens. The filter is preferablylocated on the end of GRIN lens closest to the photonics device 15, asshown in FIG. 8a. In a second, presently preferred embodiment, thefilter 17 is located near the Fourier plane B--B, which can beaccomplished by disposing the filter on a thin transparent mediapositioned near the Fourier plane, as shown in FIG. 8b.

In an additional embodiment, illustrated in FIG. 8c, the filter 17 canbe located on the photonics device 15. It will be appreciated that ifthe photonics device 15 is a micromechanical device, such as the opticalmodulator 15a, it is preferable not to locate the filter 17 on thephotonics device 15. Among other reasons, the additional mass of thefilter 17 would decrease modulator operating speed.

It is preferable to use the filter 17 in conjunction with the photonicspackage 1a, rather than the package 1b having dual microlenses 12a, 12b.If used in conjunction with the package 1b, the filter 17 should bedisposed as close as possible to the image plane, i.e., the photonicsdevice 15. If the photonics device 15 is a semiconductor device, thefilter should be disposed on it.

In an additional embodiment, the photonics device 15 can be a modulatorfor modulating the optical phase or polarization of the optical signal,with or without modulating its amplitude. As is known in the art, phasemodulation can be used to encode information onto the optical signal orto suppress undesirable effects such as stimulated Brillouin scatteringthat may occur in transmission media such as optical fiber. Since phasemodulation can be achieved by varying optical path length, thepreviously-described micromechanical modulator 15a can function as areflective phase modulator, such as by coating the moving membrane witha metal or another reflective material.

Photonics packages according to the present invention can beadvantageously used in optical communications systems, such as thepassive optical network (PON) 60 shown in FIG. 9. See U.S. patentapplication Ser. No. 08/333,926, by T. E. Darcie, N. J. Frigo and P. D.Magill, assigned to the present assignee.

The exemplary PON 60 includes a central office or head end terminal 62that has an active optical source 64, such as a multi-frequency laser orlight emitting diode (LED). The central office 62 sends information viaan optical signal 66, in wavelength-division-multiplexed (WDM) format,to a plurality of optical network units (ONU) 72. Each ONU 72 receivessuch information on a prescribed wavelength λ_(n). A wavelength routingdevice 70 demultiplexes or resolves the optical signal 66 into itsspectral components 66^(1-N), and routes each of such spectralcomponents to the appropriate ONU 72, wherein the spectral componenthaving a wavelength matching the prescribed wavelength of the ONU isrouted to that ONU.

Each ONU 72 includes a photonics package 76, such as the illustrativepackages 1a or 1b described above. In a preferred embodiment, thepackage 1a or 1b includes the combined optical modulator/photodetector15c or WLI optical modulator/photodetector 15d.

Thus, the appropriate spectral component 66^(i) of the optical signal 66is received by the photonics package 76 in an ONU 72. Typically,information is sent in "packets" via the optical signal 66. Each packetcontains a portion of downstream information for processing, as well asa portion of continuous-wave (CW) light or "optical chalkboard" uponwhich upstream information can be encoded.

As described above, the modulator portion of the device 15c or 15d isplaced in at least partially transmissive mode so that a portion of theenergy of the spectral component 66^(i) is received by the photodetectorportion of the device 15c or 15d. Thus, the optical energy reaching thephotodetector portion is converted to an electrical signal,representative of the downstream information, and routed to processingelectronics. During a second time period, the optical modulator portionencodes upstream information on the optical chalkboard portion of thepacket, and returns upstream-information-carrying spectral component67^(i), which is received by the fiber 9.

The plurality of upstream-information-carrying spectral components47^(1-N) returned from the ONUs 72 are multiplexed by the wavelengthrouting device 64 into a signal 47, which is routed to a receiver 68 inthe central office 62.

Although a number of specific embodiments of this invention have beenshown and described herein, it is to be understood that such embodimentsare merely illustrative of the many possible specific arrangements thatcan be devised in application of the principles of this invention.Numerous and varied other arrangements can be devised in accordance withthese principles by those of ordinary skill in the art without departingfrom the scope and the spirit of the invention.

We claim:
 1. A passively alignable photonic package comprising:a dualfiber ferrule comprising a retaining member having a longitudinal boretherethrough disposed along a longitudinal symmetry axis thereof, thebore receives a first and a second optical fiber in tight fittingcontact therein, each optical fiber having an optical core and acladding layer, the bore having a shape that symmetrically offsets theoptical cores from the longitudinal symmetry axis; a photonics componentcomprising a photonics device disposed on a device mount; an imagingdevice disposed between the dual fiber ferrule and the photonicscomponent, wherein the imaging device places the first and secondoptical fibers and the photonics device in optical communication; and asleeve for receiving the dual fiber ferrule, the imaging device and thephotonics component, the sleeve having a size suitable for passivelyaligning the optical fibers and the photonics component.
 2. Thephotonics package of claim 1 wherein, said first and second opticalfibers are single-mode fibers.
 3. The photonics package of claim 2,wherein the first optical fiber delivers a first optical signal.
 4. Thephotonics package of claim 3, wherein the photonics device is operableto receive the first optical signal delivered by the first optical fiberand to deliver a second optical signal to the second optical fiber. 5.The photonics package of claim 4, wherein the second optical signal is amodulated version of the first optical signal.
 6. The photonics packageof claim 3 wherein the imaging device is positioned so that itcollimates the first optical signal.
 7. The photonics package of claim1, wherein the retaining member and the sleeve comprise ceramic.
 8. Thephotonics package of claim 1 wherein the imaging device is selected fromthe group consisting of a graded index lens, a ball lens and a moldedlens.
 9. The photonics package of claim 1 wherein the photonics deviceis located in a Fourier plane relative to the imaging device and thefirst and the second optical fiber.
 10. The photonics package of claim 1further comprising a wavelength filter.
 11. The photonics package ofclaim 1 wherein the photonics device is selected from the groupconsisting of a transmitter, receiver and a transceiver.
 12. Thephotonics package of claim 11, wherein the transmitter comprises anoptical modulator.
 13. The photonics package of claim 11, wherein thereceiver comprises a photodetector.
 14. The photonics package of claim11, wherein the the transceiver comprises an optical modulator and aphotodetector in optical communication.